Highly efficient multiphoton absorption in a new quadrupolar heterocyclic dye

The multiphoton absorption properties of an organic push-molecule are investigate with nano- (ns) and femto-second (fs) laser sources. The active molecule is a new quadrupolar dye, named PEPEP, where heterocyclic units act as strong donor-acceptor moieties. Z-scan and pump-probe experiments (both single-color and with white light pulses) allow us to measure the value and the wavelength dispersion of the cross section for instantaneous two photon absorption (TPA). When Z-scan experiments are performed with nanosecond pulses, much larger effective cross sections are measured than with femtosecond pulses and with remarkably different wavelength dispersion. This is interpreted as due to two-photon absorption followed by one-photon absorption from the excited state (TPA-ESA). The TPA (or TPA-ESA) spectrum is measured, through pump-probe and z-scan measurements in a large visible range, in order to find the peak wavelength for multiphoton absorption